docs: fix typo

[jsejcksn]

Minor grammar fix

If you accept this PR, you might want to edit the repo's About description to match the change.

Comparing omission of parenthetical modifiers:

Before

• Store enhancer that syncs (a subset) of your Redux store state to localstorage.
• Store enhancer that syncs of your Redux store state to localstorage.

After

• Store enhancer that syncs (a subset of) your Redux store state to localStorage.
• Store enhancer that syncs your Redux store state to localStorage.